This invention relates to a protective tube for a temperature sensor, such as a thermocouple assembly, which is used for measuring temperatures of a material, such as a molten metal.
It is often necessary to measure the temperature of molten metals, such as aluminum and steel, during production or other industrial processes. The temperatures encountered in these operations are quite high, typically ranging from 660.degree. C. to 1540.degree. C. and higher. In addition, the corrosive nature of the metallurgical processes involved present problems which must be addressed in the design of protective apparatus for immersible temperature sensors. Materials such as slag and the molten metal encountered in various metallurgical processes are corrosive and abrasive and can dissolve and erode protective devices used with immersible temperature sensors. Further, certain protective devices do not provide satisfactory transmission of heat from the molten metal to a temperature sensor (e.g., a thermocouple assembly). Poor thermal conductors used in construction of such protective devices may substantially retard the transmission of heat to the sensor such that a relatively long period of time is required for temperature measurement, and the response time of the system to changes in material temperature is slow.
Thus, there exists a need for a protective device for a temperature sensor which provides for fast thermal response of the sensor to temperature changes, and which also exhibits good resistance to corrosion and erosion by molten materials. Additionally, it is also highly desirable to provide such a protective device having a relatively simple structure which is easy to manufacture, and which may be produced from relatively inexpensive and readily available materials.
An object of the present invention is to provide a protective tube for protecting a temperature sensor, such as a thermocouple assembly, which is used for making intermittent or continuous temperature measurements in a molten material.
A further object of the present invention is to provide a protective tube which protects a temperature sensor from a hostile molten metal environment, and which does not significantly corrode or erode with continuous or repeated use.
Yet another object of the present invention is to provide a protective tube for a temperature sensor which allows the temperature sensor to respond rapidly to temperature changes of the material whose temperature is to be measured.
It is a further object of the present invention to provide a protective tube for a temperature sensor which is relatively simple and inexpensive to manufacture, and which is reliable in its operation.
Thus, an overall object of the present invention is to provide a protective tube for protecting a temperature sensor, such as a thermocouple assembly, which balances resistance to corrosion and erosion by the materials being measured, overall response time of the temperature sensing assembly, and simple and inexpensive construction to achieve an improved overall design.
These objects and other objects of the invention are achieved by providing a protective device which has an inner tube element, preferably formed of metal, which is surrounded by refractory ceramic fiber material elements on a first immersible portion thereof. The refractory ceramic fiber material elements are preferably bonded to the inner tube element. A thermally conductive plug, which includes an inner bore portion for receiving a temperature sensor, is joined to the end of the immersible portion of the inner tube element, thereby closing the otherwise open immersible end of the tube element. At least a portion of the thermally conductive plug element is exposed for contacting a material having a temperature to be measured.
The invention provides rapid and accurate thermal measurement when the immersible end of the protective tube assembly is immersed in the material through the thermally conductive plug element which receives and substantially surrounds the temperature sensor. The inner metal tube element is protected from the material (e.g., molten metal) by the refractory ceramic fiber elements. At least a portion of the thermally conductive plug element is exposed to the material undergoing measurement, such that rapid and accurate temperature measurements are achieved. Since the thermally conductive plug element need only be provided at the end area of the inner tube element, a relatively inexpensive arrangement is provided.
According to advantageous features of certain preferred embodiments of the invention, the refractory ceramic fiber elements surround at least a portion of an outer surface of the thermally conductive plug element, in addition to the immersible end of the inner metal tube element. However, at least a portion of the thermally conductive plug element remains exposed for establishing direct contact with the material in which it is to be immersed.
According to other advantageous features of certain preferred embodiments of the invention, the bonding material used to bond the refractory ceramic fiber elements to the inner metal tube element is refractory cement. A substantially continuous layer of refractory cement may be provided between the outer surface of the inner tube and the inner surface of the ceramic fiber element or, alternatively, these components may be spot-bonded together using the refractory cement to reduce assembly time and material costs.
In certain preferred embodiments, the refractory ceramic fiber elements are formed as individual refractory tube sections which abut one another, and which are placed around the inner metal tube element. The refractory tube sections are preferably preformed in sections that are approximately twelve inches long. These embodiments provide for increased simplicity in forming the protective tube. The preformed refractory tube sections slide over the inner metal tube element and, according to certain preferred embodiments, are bonded to the inner metal tube element by refractory cement.
According to other advantageous features of certain preferred embodiments of the invention, the thermally conductive plug element extends outwardly from the end of the inner metal tube element, such that the inner bore of the thermally conductive plug element also extends out from the inner metal tube element. Thus, the thermocouple assembly, which is received in the bore of the conductive plug element, extends out from the inner metal tube element and the refractory material covering the inner metal tube element.
According to other advantageous features of certain preferred embodiments of the invention, refractory tape is applied over abutting sections of the individual refractory tube sections to seal the joints created by this construction. In other embodiments, refractory tape can also be applied over an abutting area of the thermally conductive plug and the adjacent refractory ceramic fiber element.
According to other advantageous features of certain preferred embodiments of the invention, the thermally conductive plug element is formed of graphite or silicon carbide material.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.